Cover for metal baths



Pl'l 13, 1937 c. G. FUNK E-r AL 2,076,712

covER Fon MTAL BATHs' Filed Jan. 12, 1935 l* I use 2SAM. l l HMINVENTORS.

ATTORNEYS Patented Apr. 13, 1937 COVER, FOR METAL BATES Colin G. Fink,New York, N. Y., Frederic J. Kenny, Tenaiiy, N. J., and .lohn dH. Hurd,New York, N. Y., assignors, by mesne assignments, to Spowers ResearchLaboratories, Inc., .Hersey City, N. J., a corporation of New JerseyApplicationf January 12, 1935, Serial No. L'id 5 Claims.

This invention relates to a'molten or liquid cover or blanket forapplication to the top of metal baths such as zinc baths commonlyemployed for galvanizing iron.' While the invention has been developedin connection with such zinc baths, and while it is particularly usefulin this connection, it may also find use in connection with othersimilar metal baths.

When a zinc bath is employed for galvanizlng,

l0 it is commonly maintained at atemperature in the order of 420 C to475 C., or even somewhat higher, and the tendency of the zinc underthese conditions is to rapidly oxidize with the result that a largeproportion of the zinc is lost. Even 15 as much as 50 pounds of zinc persquare foot of '30 more disadvantages, among which are the rapidvolatilization of certain covers at operating temperatures, thenecessity for keeping certain other types of covers in amoist condition,objectionable reactions between certain covers and the bath, and thehigh cost and frequent replacement required incident to the use ofcertain types of covers.

The prime feature of the present invention is,

or similar. metal baths which shall avoid these disadvantages.

A more specific feature of this invention is to provide a cover which iscomposed of inorganic material molten at the temperature of the bath,and which is capable of preventing oxidation of the zinc.Advantageously, the cover shall oat on the bath and at the same timehave a consistency which prevents access of air to the bath.`

Still another feature of theinvention resides in the provision of suchacover which does not decompose or volatilize at the temperaturesemployed, or which decomposes or volatilizes so slowly that the loss,for practical purposes, is not objectionable. i

Still another feature ofthe invention resides in the production of acover or blanketpof the character indicated which is effective andlasting in Vuse, and not excessively expensive. A

An additional feature of the invention is to provide a process forpreparing and employing a cover of this character.

Other features, objects and advantages of the invention will in part bepointed out and in part become apparent in connection Withthe followingdetailed description of o ne example of the invention, reference beinghad to the accomtherefore, to provide an improved cover for zinc- (Cl.iii-70.2)

sible variations in the preferred character 'of the invention, andwherein:

Fig. 1 is a perspective view of a solid, prismatic-shaped bodyindicating percentages of ingradients which may be employed to producecompositions which melt at certain constant temperatures; and

' Fig. 2 is a ternary diagram also illustrating, in a somewhat dierentmanner, the proportions of certain compositions which melt attemperatures indicated.

In its broader aspects, the invention consists of a cover or blanketcomposed primarily of a, normally solid substance which melts attempera- 4 tures below that of the bath oniwhich the cover is employedand which'does not volatilizeor decompose appreciably at the temperatureof the bath. We have found that excellent results may be achieved by theemployment of a salt and, more particularly, by the employment of amixture of salts which have foregoing properties and, in addition, havethe ability to form a cover of suiiicient consistency to prevent theaccess of air to the molten bath beneath the cover.

By way of example, we have found that a mixture of a chloride of analkali metal with a chloride of the' second group in the periodic tableof elements' having a comparatively low melting point, constitutes aparticularly effectiveV and satisfactory blanket. The mixture of thesalts employed is preferably such that a sufficient quantity of a salt,for instance, zinc chloride, is employed in combination with sodiumchloride'to produce a mixture having a meltingl point lower than themelting point of sodium chloride alone. If desired, a eutectic mixturemay b'e employed, although it is not in all cases necessary or advisableto employ a strictly eutectic mixture. That is to say, the mixtureshould be such that the melting point of the combined salts issufficiently low to enable the mixture to `become Vmoltenat temperaturesat or below the temperature of ,the bath. Moreover, the temperatures ofdecomposition or of volatilization -of the. several ingredients shouldbe considerably above the temperature of the molten bath upon which themixture is placed as a cover.

While the chlorides and zinc and sodium have been mentionedspecifically, it will be understood that it may be feasibletosubstltutefor` these salts other compounds or salts which are y stableat the operating temperatures and which,

at the same time, are capable of melting at la temperature below thetemperature of the bath on which the material is employed as a cover.

However, the chlorides of metals of the groups indicated have been foundto be particularly stableA and satisfactory for producing a cover whichoats upon a moltenl metal bath and '40 may contain the following:

which is capable of lasting over a considerable period of time.

As to the matter of proportions of the ingredients, it may be said ingeneral that the 5 mixture is desirably such that its melting point is-.within the range of customary temperatures employed for the bath. Byproperly adjusting the proportions,.the melting point of the cover maybe reduced even further, or, on the other hand, the bath may be employedat a temperature somewhat in excess ofnormal.

For instance, if a mixture of equal parts by weight of zinc chloride andsodium chloride is employed,-the melting point of'the mixture will beabout 532 C. However, for higher percentages of zinc chloride ascompared to sodium chloride, the melting point of the mixtures will beeven lower.

instead of the larger quantity of zinc chloride a proportion of alkalimetal chloride which will result in a mixture yhaving a melting-point-below the normal melting point of the bath.

Similarly, if a mixture of equal parts by weight of zinc chloride andpotassium chloride is employed; the melting point of the mixture will beabout 435 C. within the temperature ranges commonly employed forgalvanizing baths.

However, while satisfactory results .may be produced by employing a zincsalt in combination with a single alkali metal chloride, even superiorresults are produced by the employment of a zinc salt in combinationwith a mixture of sodium and potassium chloride, for when sodium andpotassium chloride are employed together, the latter in relatively smallamount as compared to the former, the melting point of the mixture maybe still lower. For instance, an excellent mixture merely by way ofexample,

Parts by weight Components Sodium chloride 31 Potassium chloride 17 Zincchloride 52 While indicative proportions of the ingredients have beenmentioned, other satisfactory and preferred ranges of the ingredientsare shown in the accompanying drawing. The two figures in the drawingillustrate in somewhat different manners the temperatures producible byvarious percentages of the three ingredients mentioned. Fig. 1represents a solid body of prismatic form, the three corners of theprism being represented by 100% zinc chloride; 50% zinc 55, ehloride'and50% sodium chloride; and 50% f zinc chloride and 50% potassium chloride.Constant temperature lines are indicated as lines parallel to the baseof the prism while points on the shaded surfaces represent, by theirdis- 60 tances from the respective corners, proportionsof the respectiveingredients.

With regard to Fig; 2, which is a ternary diagram, it may be noted thatthe individual lines each represent 5% by weight of given ingredi- 65ents, the right and left-hand sides of the tri- However, for reasons .ofeconomy.' and for other reasons, it is preferable to employ cated uponthe ternary diagram are -given in degrees centigrade.

If it be desired to employ `a eutectic mixture, this mixture isrepresented by the point e and has a melting point of 171 C.1 In bothFig. 1 5 and Fig. 2 there are shown certain lines e-a, e-b and e--cwhich radiate from the eutectic point and indicate the minimum meltingpoint for a mixture containing a constant proportion of one ingredientand a variation in the relative 10 proportions of the two otheringredients.

In general, we have found that 4a mixture of about 40-95 parts by weightof zinc chloride with a quantity of potassium chloride or sodiumchloride or both, to make up 100 parts by weight, 15 gives asatisfactory mixture. However, we prefer to have the zinc salt presentin proportions around 5060% by weight of a mixture of the same withsodium chloride or potassium'chloride or both. Moreover, as has beenmentioned, the 20 zinc chloride may be further increased at the expenseof the alkali metal chloride'or chlorideswhere an even lower meltingpoint is desired. Furthermore, it will be noted that sodium chloridetends to produce a somewhat 25 higher melting point than potassiumchloride. However, variations in the relative proportions lmay be madeto produce a mixture having a melting point within the range of 'about420 C.475 C., or, on the other hand, the 30 temperature of the bath maybe slightly raised tov enable the`use of somewhat higher percentages ofsodium chloride. Also where higher melting points are not objectionable,then the proportion of zinc chloride may be. somewhat 35 reducedalthough we prefer to employ a mixture containing at least 40% of zincchloride or similar salt.

' When magnesium chloride is employed in addition to the threeingredients discussed, this 40 may be used in amounts ranging from 530%by weight of the mixture of the four ingredients, the other three beingemployed generally in the relative 'proportions indicated. iIhemagnesium chloride alone has a fairly high-melting point, but 45 it mayalso tend to reduce the melting point of the mixture, and in thisrespect it may be regarded as a possible substitute for at least aportion of the zinc chloride. However, we do not regard the useofmagnesium chloride to be '50 in all cases essential, although it doesappear to improve the sheen of the galvanized product somewhat,particularly where this product is drawn through the cover. Whether thisbe due to the fact that magnesium chloride or its magnesia works intothe galvanized coating to some extent, or whether this result isattributable to catalytic eilect, we are not prepared to say. Asillustrative example of a composition which has produced excellentresults-we give the following:

Components Parts by weight Sodium chloride (NaCl) A26.4 Potassiumchloride (KCl) s 14.5 Zinc chloride (ZnClz) 43.3

Magnesium chloride (MgCl2) 15.8. (35

However, while speciiic prODrtions are indicated the followingadditional possible variations are suggested in the .nature of thecomposition. For instance, the ratio of sodium chloride to potassiumchloride may be varied within limits determined in part by therelativecost of the two materials, and also -by the temperature at which avmixture of the ingredients should melt, a1- 75lthough the major portionof the alkali metal chlorides is preferably sodium chloride. In general,we have found that excellent results (in connection with zinc baths) maybe produced when the ratio of the zinc chloride to sodium chloride topotassium chloride is about 3 to 2 to 1 by weight, whether or not othermaterials are also added. Our research has appeared to show thatvariations of at least plus or minus 10% inthe quantities of these saltsare feasible and satisfactory, but in the broader aspects of the presentdevelopment, other variations are contemplated.

As stated above, the use of magnesium chloride is not consideredessential to the production of a satisfactory cover for vzinc baths, butto some extent at least it may be considered as a substitute for zincchloride although it does not produce as low a resulting temperature asdoes the latter when mixed with alkali metal chlorides. It may also havethe effect of making the cover more of the proper consistency forexcluding air from lthe bath. Y

While the particular salts named have been found especiallyadvantageous, it will be understood that the invention in its broaderaspects is not limited to these details.

4In preparing the mixture to be employed as a cover, we preferably mixthe ingredients in finely divided form and then melt the mixture at atemperature of around 500 C. The mixture given in the second table willcommence to melt at around 390 C., but we prefer to heat it to thehigher temperature mentioned because this appears to produce a superior,uniform composition. During the heating some sediment may form, and itis desirable to continue the heating until the liquid is clear. Theliquid may be drawn off from the sediment and passed directly to thebath which is to be covered, or the liquid may be allowed to solidifyand may be put up in any suitable form for marketing to concernsdesiring this type of product.

The thicknessof the material employed as a cover on the bath may varyconsiderably but ordinarily a cover of less than an inch in thicknessproduces satisfactory results. Those skilled in the art will understandthe limits within which the thickness may vary, and they can readilydetermine, taking into account considerations of economy, the desiredamount of cover to use in order to reduce oxidation of the bath to asatisfactory point.

Should the bath, after a considerable time, become less effective, smalladditional quantities 1 of zinc chloride may be added. iHowever, it ispreferable when the cover eventually loses its efiiciency, to remove it,cool it and remelt it together with suiiicient additional ingredients tomake up the desired proportions.

It is also possible in the employment of a cover of this type tointermingle therewith certain additional materials of solid character,such as charcoal or slag from iron furnaces. Such materials appear toaid in prolonging the life of the wipes of galvanizing baths.-Preferably, the proportions of these solid ingredients are notsufiiciently large to alter the liquid character of the cover, for aliquid cover appears to present great advantages over covers formed ofparticles of solid material.

Through the present invention, therefore, there is produced a cover,which not only prevents the access of air to the bath, but also doesnot require any material attention during the use of the bath. Inotherwords, the cover is not destroyed or adversely affected by thetemperature of the bath but remains in molten form with sufiicientcontinuity, density or viscosity, to form an eiective seal. At the sametime, the cover does not adversely aect the character of the zinc bathand does not tend to alloy with the container for the bath, or stick tothe surface thereof. Furthermore, inasmuch as the cover lasts a longtime, the initial cost of the same is not excessive.

The terms and expressions which have been employed are used as terms ofdescription and not of limitation, and there is no intention, in theusev of such terms and expressions, of excluding any equivalents of thefeatures shown and described, or portions thereof, but it is recognizedthat various modiiications are possible within the scope of theinvention claimed.

We claim:

1. A method of preventing oxidation of a zinc bath which comprisesapplying thereto a mix-Y ture containing zinc chloride, sodium chlorideand potassium chloride in the relative proportions of about 3 to 2 to l,together with magnesium chloride.

2. A cover for zinc baths comprising a mixture of inorganic salts havinga melting pointA below the melting point of the bath and beingnon-volatile and stable at the temperature of the bath, said mixturecomprising about 26% of sodium chloride, about 14% of potassiumchloride, about 43% of zinc chloride and about l15% of magnesiumchloride.

3. A molten bath blanket comprising a mixture of 40-95 parts by weightof zinc chloride with about 5 to 60 parts by weight of sodium andpotassium chlorides, the latter being present in a relatively smallamount as compared to the former, together with magnesium chloride.

4. Process of galvanizing which comprises providing a bath of moltenzinc at a temperature of about 420 C. to about 475 C., dippingthelarticles to be galvanized into said hot bath, and maintaining amolten blanket of substantial thickness upon the surface of said zincbath, said blanket comprising a mixture of zinc chloride, sodiumchloride and potassium chloride, the zinc chloride constituting themajor ingredient of the mixture, the sodium chloride in the order ofone-quarter to a third of the mixture and the potassium chloride insubstantial amount but less than the sodium chloride, a substantialproportion of magnesium chloride, said mixture having a melting pointsubstantially lower than the melting point of the zinc bath and beingcapable of forming a stable, substantially air-excluding cover over thesurface of the zinc bath for substantial operating periods of timewithout decomposing or vaporizing.

5. A cover for zinc baths and for wiping the galvanized articles passingtherethrough, comprising a mixture of inorganic salts having a meltingpoint below the melting point of the bath and being non-volatile andstable at the temperature of the bath `and acting to remove impuritiesand scum after galvanizing, said mixture containing zinc chloride,sodium chloride, and potassium chloride in the relative proportions ofabout 3 to 2 to l, together with magnesium chloride.

COLIN G. FINK.

FREDERIC J. KENNY. JOHN nI- I. HORD.

